Activating V-Raf murine sarcoma viral oncogene homolog B1 (BRAF) mutations are prevalent in numerous types of cancers, including 50-70% of melanomas, 15% of colorectal and ovarian cancers, and 36-69% of papillary thyroid carcinomas (reviewed in Davies et al., 2002, Nature, 417:949-954; and Namba et al., 2003, J. Clin. Endocr. Metab., 88:4393-97). Activating BRAF mutations have also been identified in up to 82% of benign melanocytic tumors (nevi) (Pollock et al., 2003, Nature Genet., 33:19-20). The most common activating BRAF mutation is a glutamic acid to valine substitution at position 600 (V600E; formerly identified as V599E). This mutation produces a highly active kinase that stimulates constitutive extracellular signal-regulated protein kinase (ERK) signaling. Expression of BRAFV600E has been shown to induce senescence in cultured human fibroblasts (Zhu et al., 1998, Genes Dev., 12:2997-3007) and human melanocytes (Michaloglou et al., 2005, Nature, 436:720-724) and in vivo in preneoplastic nevi (Michaloglou et al., 2005, Nature, 436:720-724).